Electrochemical capacitors (ECs), also known as ultracapacitors or supercapacitors, are being considered for uses in hybrid electric vehicles (EVs) where they can supplement a battery used in an electric car to provide bursts of power needed for rapid acceleration, the biggest technical hurdle to making battery-powered cars commercially viable. A battery would still be used for cruising, but capacitors (with their ability to release energy much more quickly than batteries) would kick in whenever the car needs to accelerate for merging, passing, emergency maneuvers, and hill climbing. The EC must also store sufficient energy to provide an acceptable driving range. To be cost- and weight-effective compared to additional battery capacity they must combine adequate specific energy and specific power with long cycle life, and meet cost targets as well. Specifically, it must store about 400 Wh of energy, be able to deliver about 40 kW of power for about 10 seconds, and provide high cycle-life (>100,000 cycles).
ECs are also gaining acceptance in the electronics industry as system designers become familiar with their attributes and benefits. ECs were originally developed to provide large bursts of driving energy for orbital lasers. In complementary metal oxide semiconductor (CMOS) memory backup applications, for instance, a one-Farad EC having a volume of only one-half cubic inch can replace nickel-cadmium or lithium batteries and provide backup power for months. For a given applied voltage, the stored energy in an EC associated with a given charge is half that storable in a corresponding battery system for passage of the same charge. Nevertheless, ECs are extremely attractive power sources. Compared with batteries, they require no maintenance, offer much higher cycle-life, require a very simple charging circuit, experience no “memory effect,” and are generally much safer. Physical rather than chemical energy storage is the key reason for their safe operation and extraordinarily high cycle-life. Perhaps most importantly, capacitors offer higher power density than batteries.
The high volumetric capacitance density of an EC (10 to 100 times greater than conventional capacitors) derives from using porous electrodes to create a large effective “plate area” and from storing energy in the diffuse double layer. This double layer, created naturally at a solid-electrolyte interface when voltage is imposed, has a thickness of only about 1-2 nm, thus forming an extremely small effective “plate separation.” In some ECs, stored energy is further augmented by pseudo-capacitance effects, occurring again at the solid-electrolyte interface due to electrochemical phenomena such as the redox charge transfer. The double layer capacitor is based on a high surface area electrode material, such as activated carbon, immersed in an electrolyte. A polarized double layer is formed at electrode-electrolyte interfaces providing high capacitance.
Experience with ECs based on activated carbon electrodes shows that the experimentally measured capacitance is always much lower than the geometrical capacitance calculated from the measured surface area and the width of the dipole layer. For very high surface area carbons, typically only about ten percent of the “theoretical” capacitance was observed. This disappointing performance is related to the presence of micro-pores and ascribed to inaccessibility of some pores by the electrolyte, wetting deficiencies, and/or the inability of a double layer to form successfully in pores in which the oppositely charged surfaces are less than about 2 nm apart. In activated carbons, depending on the source of the carbon and the heat treatment temperature, a surprising amount of surface can be in the form of such micro-pores.
It would be desirable to produce an EC that exhibits greater geometrical capacitance using a carbon based electrode having a high accessible surface area, high porosity, and reduced or no micro-pores. It would be further advantageous to develop carbon-based nano-structures that are conducive to the occurrence of pseudo-capacitance effects such as the redox charge transfer.
In this context, carbon nanotubes (CNTs) are of great interest. CNTs are nanometer-scale sized tube-shaped molecules having the structure of a graphite molecule rolled into a rube. A nanotube can be single-walled or multi-walled, dependent upon conditions of preparation. Carbon nanotubes typically are electrically conductive and mechanically strong and stiff along their length. Nanotubes typically also have a relatively high aspect ratio (length/diameter ratio). Due to these properties, the use of CNTs as reinforcements in composite materials for both structural and functional applications would be advantageous. In particular, CNTs are being studied for electrochemical supercapacitor electrodes due to their unique properties and structure, which include high surface area, high conductivity, and chemical stability. Capacitance values from 20 to 180 F/g have been reported, depending on CNT purity and electrolyte, as well as on specimen treatment such as CO2 physical activation, KOH chemical activation, or exposure to nitric acid, fluorine, or ammonia plasma. Carbon nano-fibers (CNFs) and graphitic nano-fibers (GNFs), two thicker-diameter cousins of CNTs, have also been investigated as potential EC electrode materials.
Conducting polymers, such as polyacetylene, polypyrrole, polyaniline, polythiophene, and their derivatives, are also common electrode materials for supercapacitors. The modification of CNTs with conducting polymers is one way to increase the capacitance of the composite resulting from redox contribution of the conducting polymers. In the CNT/conducting polymer composite, CNTs are electron acceptors while the conducting polymer serves as an electron donor. A charge transfer complex is formed between CNTs in their ground state and aniline monomer. A number of studies on CNT/conducting polymer composites for electrochemical capacitor applications have been reported. The following references [Refs. 1-8] are related to CNT-, CNF-, or GNF-based EC electrodes:    1. K. H. An, et al., “Electrochemical Properties of High-Power Supercapacitors Using Single-Walled CNT Electrodes,” Advanced Functional Materials, 11 (No. 5) (October 2001) 387-392.    2. G. Z. Chen, “Carbon Nanotube and Polypyrrole Composites: Coating and Doping,” Advanced Materials, 12 (No. 7) (2000) 522-526.    3. C. Zhou, et al., “Functionalized Single Wall CNTs Treated with Pyrrole for Electrochemical Supercapacitor Membranes,” Chemistry of Materials, 17 (2005) 1997-2002.    4. K. Jurewicz, et al., “Supercapacitors from Nanotubes/Polypyrrole Composites,” Chemical Physics Letters, 347 (October 2001) 36-40.    5. J. E. Huang, et al., “Well-dispersed Single-walled CNT/Polyaniline Composite Films,” Carbon, 41 (2003) 2731-2736.    6. H. Tennent, et al., “Graphitic Nano-fibers in Electrochemical Capacitors,” U.S. Pat. No. 6,031,711 (Feb. 29, 2000).    7. H. Tennent, et al., “High Surface Area Nanofibers, Methods of Making, Methods of Using and Products Containing Same,” U.S. Pat. No. 6,099,960 (Aug. 8, 2000).    8. C. M. Niu, “Fibril Composite Electrode for Electrochemical Capacitors,” U.S. Pat. No. 6,205,016 (Mar. 20, 2001).    9. R. A. Reynolds, III, “Method of Making Composite Electrode and Current Collectors,” U.S. Pat. No. 6,830,595 (Dec. 14, 2004).    10. R. A. Reynolds, III, et al., “Double-Layer Capacitor Component and Method for Preparing Them,” U.S. Pat. No. 6,757,154 (Jun. 29, 2004).    11. R. A. Reynolds, III, “Composite Electrode and Current Collectors and Processes for Making the Same,” U.S. Pat. No. 7,206,189 (Apr. 17, 2007).
However, there are several drawbacks associated with carbon nano-tubes or nano-fibers for EC electrode applications. First, both nano-tubes and nano-fibers are extremely expensive. Second, both materials tend to form a tangled mess resembling a hairball, which is difficult to work with. These and other difficulties have limited efforts toward commercialization of supercapacitors containing nano-tube or nano-fiber based electrodes.
As a less expensive material, flexible graphite sheet has been used in an integrated electrode/current collector for EC applications, wherein the flexible graphite sheet is used as a substrate to support thereon an electrode active material (e.g., activated carbon particles) [Refs. 9-11]. Actually, these carbon particles are embedded on the surface or into the bulk of a flexible graphite sheet. The “flexible graphite” is typically obtained by first treating natural graphite particles with an intercalating agent (intercalant) that penetrates into the inter-planar spacings of the graphite crystals to form a graphite intercalated compound (GIC). The GIC is then exposed to a thermal shock, up to a temperature of typically 800-1,100° C., to expand the intercalated particles by typically 80-300 times in the direction perpendicular to the graphene layers (basal planes) of a graphite crystal structure. The resulting expanded or exfoliated graphite particles are vermiform in appearance and are, therefore, commonly referred to as graphite worms. Hereinafter, the term “exfoliated graphite” will be used interchangeably with the term “expanded graphite.” The worms may be re-compressed together into flexible sheets which can be formed and cut into various shapes. These thin sheets (foils or films) are commonly referred to as flexible graphite. Flexible graphite can be wound up on a drum to form a roll of thin film, just like a roll of thin plastic film or paper. The flexibility or compressibility of flexible graphite or exfoliated graphite enables the hard solid carbon particles to be embedded into the flexible graphite sheet when solid carbon particles and exfoliated graphite are combined and calendared, roll-pressed, or embossed together. However, such a combined electrode/current collector as disclosed in [Refs. 9-11] has several major shortcomings:    1. The exfoliated graphite or flexible graphite sheet cited in these patents is a passive material that is used solely as a substrate or binder material to hold the electrode active material together for forming an integral member (electrode/current collector). The flexible graphite or exfoliated graphite itself is not used as an electrode active material, i.e., it does not provide the diffuse double layer charges and, hence, does not contribute to the double layer capacitance.    2. In order for a flexible graphite sheet or exfoliated graphite particles to hold activated carbon particles together, the total amount of exfoliated graphite must be at least 50% by volume or more. Individual graphite particles are a solid, not a liquid adhesive. Although exfoliated graphite particles themselves can be re-compressed together to form a cohered body, the resulting flexible graphite sheet is normally very fragile. When a large amount of exfoliated graphite is used, the relative proportion of the electrode active material (the material that actually contributes to double layer capacitance) is small. Consequently, the effective energy density of the resulting supercapacitor is significantly curtailed.    3. By embedding activated carbon particles into a flexible graphite sheet or mixing activated carbon particles with exfoliated graphite particles, one tends to seal off the pores of activated carbon particles that have surface openings supposedly functioning to accommodate the liquid electrolyte. Mixing or embedding significantly reduces the amount of carbon particle pores that are designed to be accessible by liquid electrolyte, thereby reducing the effective electrolyte-electrode interface areas where double layer charges can be formed.    4. The activated carbon particles utilized by Reynolds, et al. [Refs. 9-11] were typically in the range of 600 μm and 900 μm. They were too big to penetrate the inter-layer spaces (<2.8 nm within an inter-planar spacing of 0.335 nm) between two graphene planes of unexpanded graphite crystallites. They were also too big to penetrate the space (typically <10 μm) between graphite flakes (each flake comprising a multiplicity of graphene sheets bonded by van der Waal's forces). With a maximum average expansion ratio of 300, the original inter-planar spacing of 0.335 nm would become at most 100 nm on average. In rare cases, there could be some pores as large as 10 μm, but these pores are still too small to accept activated solid carbon particles. In actuality, the activated carbon particles are simply squeezed by and held in place between clusters of expanded graphite flakes. Of course, such a configuration is advantageous in that it provides a substrate with good electrical conductivity and this substrate functions as a current collector as well.
Instead of trying to develop much lower-cost processes for making CNTs, researchers (Jang, et al.) at Nanotek Instruments, Inc. have worked diligently to develop alternative nano-scaled carbon materials that exhibit comparable properties, but are more readily available and at much lower costs. This development work has led to the discovery of processes for producing individual nano-scaled graphite planes (individual graphene sheets) and stacks of multiple nano-scaled graphene sheets, which are collectively called nano-sized graphene plates (NGPs). NGPs could provide unique opportunities for solid state scientists to study the structures and properties of nano carbon materials. The structures of these materials may be best visualized by making a longitudinal scission on the single-wall or multi-wall of a nano-tube along its tube axis direction and then flattening up the resulting sheet or plate. FIG. 1 shows an atomic force microscopic picture of a sample of NGPs. In practice, NGPs are obtained from a precursor material, such as minute graphite particles, using a low-cost process, but not via flattening of CNTs. One of the cost-effective processes is exfoliation of graphite to produce graphite worms of loosely connected flakes, followed by separation of these flakes into isolated (unconnected) graphene platelets. These nano materials could potentially become cost-effective substitutes for CNTs or other types of nano-rods for various scientific and engineering applications. These diligent efforts have led to the following patent applications [Refs. 12-20]:    12. B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4, 2006).    13. B. Z. Jang, L. X. Yang, S. C. Wong, and Y. J. Bai, “Process for Producing Nano-scaled Graphene Plates,” U.S. patent pending, Ser. No. 10/858,814 (Jun. 3, 2004).    14. Aruna Zhamu, JinJun Shi, Jiusheng Guo, and Bor Z. Jang, “Low-Temperature Method of Producing Nano-scaled Graphene Platelets and Their Nanocomposites,” U.S. patent Pending, Ser. No. 11/787,442 (Apr. 17, 2007).    15. Aruna Zhamu, Jinjun Shi, Jiusheng Guo and Bor Z. Jang, “Method of Producing Exfoliated Graphite, Flexible Graphite, and Nano-Scaled Graphene Plates,” U.S. patent Pending, Ser. No. 11/800,728 (May 8, 2007).    16. Aruna Zhamu, Joan Jang, Jinjun Shi, and Bor Z. Jang, “Method of Producing Ultra-thin Nano-Scaled Graphene Platelets,” U.S. patent Pending, Ser. No. 11/879,680 (Jul. 9, 2007).    17. Aruna Zhamu, Joan Jang, and Bor Z. Jang, “Electrochemical Method of Producing Ultra-thin Nano-Scaled Graphene Platelets,” U.S. patent Pending, Ser. No. 11/881,388 (Jul. 27, 2007).    18. Aruna Zham and Bor Z. Jang, “Environmentally Benign Chemical Method of Producing Exfoliated Graphite, Flexible Graphite, and Nano-Scaled Graphene Platelets,” U.S. patent Pending, Ser. No. 11/881,389 (Jul. 27, 2007).    19. Aruna Zham and Bor Z. Jang, “Environmentally Benign Graphite Intercalation Compound Composition for Exfoliated Graphite, Flexible Graphite, and Nano-Scaled Graphene Platelets,” U.S. patent Pending, Ser. No. 11/881,390 (Jul. 27, 2007).    20. Lulu Song, A. Zhamu, Jiusheng Guo, and B. Z. Jang “Nano-scaled Graphene Plate Nanocomposites for Supercapacitor Electrodes” U.S. patent Pending, Ser. No. 11/499,861 (Aug. 7, 2006).
For instance, Jang, et al. [Ref. 13] disclosed a process to readily produce NGPs in large quantities. The process includes the following procedures: (1) providing a graphite powder containing fine graphite particles preferably with at least one dimension smaller than 200 μm (most preferably smaller than 1 μm); (2) exfoliating the graphite crystallites in these particles in such a manner that at least two graphene planes are fully separated from each other, and (3) mechanical attrition (e.g., ball milling) of the exfoliated particles to become nano-scaled, resulting in the formation of NGPs with platelet thickness smaller than 100 nm. The starting powder type and size, exfoliation conditions (e.g., intercalation chemical type and concentration, temperature cycles, and the mechanical attrition conditions (e.g., ball milling time and intensity)) can be varied to generate, by design, various NGP materials with a wide range of graphene plate thickness, width and length values. We have successfully prepared NGPs with an average length in the range of 1 to 20 μm). However, the length or width can be smaller than 500 nm and, in several cases, smaller than 100 nm. Ball milling is known to be an effective process for mass-producing ultra-fine powder particles. The processing ease and the wide property ranges that can be achieved with NGP materials make them promising candidates for many important engineering applications. The electronic, thermal and mechanical properties of NGP materials are expected to be comparable to those of carbon nano-tubes; but NGPs will be available at much lower costs and in larger quantities.
After an extensive and in-depth study of the electrochemical response of NGPs and their composites, we have found that a certain class of meso-porous composites containing NGPs as electrode ingredients exhibit superior charge double layer-type supercapacitance and redox charge transfer-type pseudo-capacitance. Preferred compositions were described in an earlier application [20]. These electrode materials can be mass-produced cost-effectively and, hence, have much greater utility value compared to carbon nanotube-based materials.
Thus, it is an object of the present invention to provide a process for producing porous nanocomposites that contain fully separated nano graphite platelets (NGPs) with a sufficient amount and packing arrangement effective for achieving a high surface area greater than 100 m2/gm. The specific area of the resulting nanocomposite electrode is typically greater than 200 m2/gm and, in many cases, greater than 500 m2/gm and even greater than 1000 m2/gm when the nanocomposite matrix is made via pyrolization of a polymer.
It is another object of the present invention to provide a process for producing a supercapacitor electrode featuring porous nanocomposites that contain fully separated graphite platelets with a sufficient level of porosity effective for achieving a high capacitance value.
It is yet another object of the present invention to provide a process for continuously producing a porous nanocomposite electrode comprising fully separated graphite platelets that are smaller than 10 μm in length, width or diameter and smaller than 100 nm in thickness (typically and preferably smaller than 10 nm).
It is still another object of the present invention to provide a process for producing porous nanocomposites comprising fully separated graphite platelets that are surface-functionalized or activated.
It is still another object of the present invention to provide a process for producing a porous nanocomposite electrode comprising fully separated graphite platelets that are smaller than 100 nm (preferably smaller than 10 nm) in thickness. These nano-scaled graphene plates are attached to or bonded by a conductive material such as a conjugate chain polymer for a significantly improved charge storage capacity. The matrix material may comprise a conducting polymer, polymeric carbon, coal tar pitch, petroleum pitch, glassy or amorphous carbon, or a combination thereof